In the Features View for the site or virtual directory you want this to apply to, look for the Handler Mappings icon in the lower IIS section. Double click that to get into the actual list of handlers currently mapped. You should theoretically see existing entries for things like *.aspx. Double click the *.aspx one and use it as an example to create a new mapping for *.html. The Add Script Map in the right hand pane is what you should use for this particular entry.

You can also do this in the web.config file directly. But it's probably easier to just do it via the GUI first and then look at the resulting web.config file to see what changes were actually made.